In many laser applications it is necessary to accurately tune the output of the laser light source to a particular wavelength. For fiber optic communications in particular, accurate tuning of the communication lasers is necessary to permit adjacent transmission channels to be closely spaced, often at wavelengths differing by only 0.4 nanometers. For such closely spaced channels, a laser's wavelength must be tuned to the assigned channel with an accuracy of +/-0.1 nanometers or less. One method of accurate laser tuning uses an etalon having an appropriate thickness to discriminate between various input wavelengths. For accurate results, the thickness of the etalon must be controlled very precisely.
FIG. 1 is a illustration of a conventional etalon 10. The etalon has two partially reflective parallel surfaces 12, 14 separated by a distance d and is comprised of a material with an index of refraction r. When collimated light having a wavelength .lambda. is passed through the etalon, some of the light is reflected from the surfaces 12, 14. The multiply reflected light beams interfere, either constructively or destructively, with each other, and thus alter the overall intensity of the light which passes through the etalon 10. Maximum transmission occurs when twice the distance between the reflective surfaces 12, 14 is an integral number of wavelengths .lambda. in the etalon. In other words, 2d*r/.lambda.=x, where x is an integer. The transmission characteristics of etalon 10 are illustrated in the graphs of FIGS. 2a and 2b. As shown, the transmission characteristic is a periodic function of wavelength and the percentage of reflectivity R of the partially reflective surfaces 12, 14. The thickness d and refractive index r of the etalon determine the distance between the peaks around a given wavelength. The reflectivity R determines the percentage of the light that is reflected by the etalon walls. This defines the amount of light which is available for constructive and destructive interference, and thus how narrow or broad the transmission peaks are.
In order for an etalon to be used to tune the frequency of a light source, the intensity of the transmitted light must be determined. Typically, this is accomplished through the use of a photodetector 16. The intensity of the output light as measured by the photodetector 16 is compared to the intensity of a reference beam and the difference is used to generate an error signal that is used to adjust the wavelength of the light produced by the laser, for example, by varying the laser's operating temperature.
Optimal control over wavelength is provided when a large change in the transmission intensity of an etalon occurs over a small change in wavelength. In other words, optimal control is achieved when the slope of the transmission curve is steepest, such as region 20 shown in FIG. 2a. As the slope of the curve lessens towards a peak or trough, shown as regions 22, the transmission intensity varies to a lesser degree with changes in wavelength and the intensity measurement provides only marginal control. At wavelengths which are approximately at a peak or trough 24 of the transmission curve, the transmitted intensity varies very little with changes in wavelength and little or no useful control information is provided by the etalon. Therefore, in order for a conventional etalon to be useful in calibrating the frequency of a light input source, the distance d between the reflective surfaces must be controlled to a very high degree of accuracy so that the wavelength of interest, .lambda..sub.0, falls within a steep slope region 20 of the transmission curve.
It is not unusual for conventional applications to require the etalon thickness d to be accurate to an order of one part per million or more. Producing etalons having a thickness controlled to this level of accuracy generally results in a low manufacturing yield because even very small changes in the total thickness of the device result in large effects. In addition, the operating characteristics of an etalon are highly temperature dependent. Thus, when used, the etalon's temperature must be precisely controlled to prevent changes in thickness due to thermal expansion. In systems where the wavelength of the laser being tuned is adjusted by controlling the operating temperature of the laser, the etalon cannot be housed in the same thermal chamber as the laser because the temperature changes will also affect the transmission characteristics of the etalon. Instead, the etalon must be housed in its own thermally controlled chamber which maintains the etalon at a predetermined temperature. This increases both the cost and the complexity of an etalon-based laser tuning assembly. In addition, because the relative thickness of the etalon is dependent on the angle of the incident light, it is necessary to accurately position the etalon so that the incident light is normal to the surface, a task whose difficulty is increased when the laser and etalon are separately housed.
Alternative etalon structures are also relatively complex and expensive. For example, hollow etalons with mechanisms to physically adjust the position of one of the reflecting surfaces or multi-etalon devices with mechanisms to adjust the position of one etalon relative to another are used to provide some degree of temperature compensation. However, they are complex to build and control and each can cost tens of thousands of dollars. Similarly, wedge-shaped etalons, while providing a variety of thicknesses over the width of the wedge, are difficult to fabricate with an accurate wedge angle and devices for controlling the entry point of the light into the wedge, so as to strike an area with the desired thickness, are also complex and expensive.
Because etalons transmit light of only particular wavelengths, they are also used as spectral sensors to indicate when certain wavelengths of light are present in a given light beam. It is known to provide an etalon with a stepped surface so that a single device can be used to detect several different frequencies of light. Stepped spectrographic etalon arrangements of this type are shown in U.S. Pat. No. 4,822,998 to Yokota et al. and U.S. Pat. No. 5,144,498 to Vincent. Etalons used in spectrographic analysis are designed to discriminate across a wide range of wavelengths, typically on the order of wavelength differences of hundreds of nanometers. To accommodate this, the etalon step size must be relatively large and, as a result, the primary transmission peaks between adjacent steps are far apart, typically at least several tens of nanometers.
To allow for accurate sensing of wavelengths within the spectral range, the transmission peaks corresponding to each step must also be well defined and separated from each other to provide sharp, isolated detection signals. Because spectral sensing etalons of this type are optimized for operation across a wide range of wavelengths, they are unsuitable for high precision laser tuning applications where it is necessary to accurately tune a narrow range of laser output frequencies with accuracies of one nanometer or less.